1. Field of the invention
The present invention relates to a retardation film and a process for producing the same. The present invention also relates to an optical film as a combination of the retardation film and a polarizer or a polarizing plate. The retardation film and the optical film can be applied to an image display such as a liquid crystal display, an organic EL display, and PDP. In particular, the retardation film of the present invention is suitable for compensating for a viewing angle of a liquid crystal cell of TN mode, VA mode, IPS mode, FFS mode or OCB mode, and an optical film using the retardation film can be suitably applied to a liquid crystal panel, and a liquid crystal display.
2. Description of the Related Art
In a liquid crystal display, generally, a liquid crystal panel in which a polarizer is disposed on both sides of a liquid crystal cell is used. In addition, in the liquid crystal panel, in order to optically compensate for retardation due to birefringence of the liquid crystal cell in normal direction and in oblique direction, a retardation film is disposed between the liquid crystal cell and the polarizer. In order to improve display property of the liquid crystal display in oblique direction, how a retardation value of the retardation film varies relative to an angle in oblique direction is very important.
Then, a retardation film in which a retardation value is approximately constant not depending on an angle has been proposed (JP-A-2-160204). The retardation film shows a relationship of nx>nz>ny where the refractive indices in the slow axis direction, the fast axis direction and the thickness direction of the film to be nx, ny and nz, respectively. An optical film using such the retardation film and the polarizing plate, and the IPS mode or VA mode liquid crystal display in which the viewing angle property in oblique direction is improved using the retardation film have been also proposed (JP-A-11-305217, JP-A-2000-39610, and JP-A-4-305602).
As a process for producing a retardation film showing a relationship of nx>nz>ny, a process for producing the retardation film by adhering a shrinkable film to one side or both sides of a polymer film to give a laminate, and subjecting the laminate to heat-stretching treatment has been proposed (JP-A No. 5-157911). In this process, a refractive index distribution of the polymer film is made to greatly change before and after stretching. For this reason, as a polymer film to be used, a polymer film in which retardation is easily generated in a low stretching ratio is preferable and, usually, a polycarbonate-based resin, and an aromatic polymer film such as a polyallylate-based resin and a polysulfone-based resin have been used. However, since the aromatic polymer film has a large absolute value of photoelastic coefficient, retardation is easily changed relative to stress. For this reason, there is a problem that, when the film is exposed to a high temperature in the state where the film is arranged between the liquid crystal cell and the polarizer, a retardation value is shifted from a designed value due to a shrinking stress of the polarizer or, when it is used in the liquid crystal display, a unevenness of a retardation value is generated due to a unevenness of stress generated by heat of backlight, deteriorating display property. On the other hand, an aliphatic-based resin film such as a norbornene-based resin film has a small absolute value of photoelastic coefficient. In the aliphatic-based resin film, however, since retardation is generated with difficulty, a desired retardation value could not be obtained not only by stretching at a low stretching ratio as in an aromatic-based polymer film, but also at a high stretching ratio. In addition, when the film is stretched at a high stretching ratio, there is a problem that a film is broken.